1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to equipment utilized to planarize semiconductor wafers.
2. Background of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers above a base substrate to form the necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of a layer must be achieved in order to pattern and form various components and interconnects. In modern integrated circuit (IC) fabrication technology, it is necessary to form various structures over previous layers formed on a semiconductor wafer. With diminishing feature size such structures result in highly irregular surface topography causing manufacturing problems in the formation of thin film layers. To facilitate manufacturing processes, the rough surface topography has to be smoothened or planarized.
One of the methods for achieving planarization of the wafer surface is chemical mechanical polishing (CMP). CMP is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. CMP is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer, in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to planarize the exposed surface of the wafer (typically, a layer formed on the wafer), when subjected to a force pressing the wafer onto the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery, as well as in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more effective polishing rate is performing the CMP using the linear planarization technique. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the global planarity is improved over CMP tools using rotating pads. One such example of a linear polisher is described in U.S. Pat. No. 5,692,947.
Unlike the hardened table top of a rotating polisher, linear planarizing tools are capable of using flexible belts, upon which the pad is disposed. This flexibility allows the belt to flex, which can cause a change in the pad pressure being exerted on the wafer. When this flexibility can be controlled, it provides a mechanism for adjusting the planarization rate and/or the profile across the surface of the wafer. Therefore, a fluid support (or platen) can be used to adjust the pad pressure being exerted on a wafer at various locations along the wafer surface. An example of a fluid support is disclosed in U.S. Pat. No. 5,558,568.
When CMP is employed, it is generally advantageous to monitor the effects of the planarizing process to determine if the process is being performed according to desired specifications. Furthermore, when the polishing profile can be adjusted, such as with the use of the linear polisher (planarizing tool) described above, the monitoring of the CMP process allows the planarizing profiles to be adjusted pursuant to the measured values obtained from the monitoring. Accordingly, it would be advantageous to provide some form of monitoring during the actual polishing process or between the polishing of each wafer, so that measurements can be obtained to ascertain various polishing parameters.
A monitoring problem specific to CMP is the determination of the process end point. That is, the ability to monitor the thickness of the material being removed (or alternatively, the thickness of the material remaining). Essentially, the CMP process is monitored and the planarizing process terminated, when some desired preset material thickness parameter is achieved on the wafer surface. The end-point detection technique detects this point where the CMP process is to be stopped. There are a variety of end-point detection schemes known in the art.
For example, one technique employs an electrode, which is inserted into the polish platen in order to monitor the capacitance of a dielectric layer disposed on a wafer substrate surface (see for example, U.S. Pat. No. 5,081,421). Another technique employs the monitoring of the electrical current to a motor which rotates the wafer (see for example, U.S. Pat. Nos. 5,308,438 and 5,036,015). Other techniques using electrical current to detect the process end-point are also known. For example, the presence or absence of a metal layer can be detected by current flow (see for example, U.S. Pat. No. 4,793,895) or by using an impedance measurement (see for example, U.S. Pat. No. 5,213,655). Another technique is the use of resistance or impedance measurements by using contact structures through the wafer (see for example, U.S. Pat. No. 5,321,304). Still another technique uses an acoustic wave reflection to monitor dielectric thickness (see for example, U.S. Pat. No. 5,240,552). Optical techniques are now being implemented as an accurate indicator for measuring material thickness on a wafer (see for example, U.S. Pat. No. 5,433,651). Accordingly, it is understood that a number of techniques are available for detecting the end-point of a polishing cycle for a semiconductor wafer.
Historically, there have been two general approaches to monitoring the thickness of a material film layer residing on a wafer. The first approach is the "stand alone" scheme. In a stand alone system, the film thickness measurements are obtained after a wafer has been fully processed. The thickness measurements are obtained on a separate piece of monitoring equipment, usually disposed as a "stand-alone" equipment and not part of the CMP tool. This technique only provides a post-mortem information of the remaining film quality, which frequently results in considerable loss of manufacturing time to correct the process if the material thickness removal is out of tolerance.
The second approach is the "in-situ" detection scheme in which in-situ end-point detectors monitor the ongoing polishing process. Such in-situ detectors are usually integrated into the CMP tool itself. Although in-situ systems allow for adjustments to be made during the on-going process, the equipment usually requires extremely sophisticated algorithms to interpret and to predict the correct film thickness. Furthermore, when multi-level integrated circuit devices are being planarized, the algorithm can become significantly complex. In some instances, such complex algorithms can take considerably long time to execute, which can add appreciable time to the polishing process.
The present invention implements a CMP monitoring tool which draws on a stand alone concept, but is integrated into a cluster tool configuration to provide for monitoring of the CMP process during the processing cycle of a wafer. Instead of using an in-situ monitoring scheme, the present invention provides for discrete steps in monitoring the wafer during the CMP process.